Change the amount of fuel you are putting in the tank, also make sure you're mostly running on premium fuel (lean meats, veggies, fruits) not 87 octane (cookies, breads, pop, junk, etc.)
At 6 ft tall, 300 lbs 30 year old male you need 2600 calories a day to maintain your weight if you never move off the couch all day.
If you want to lose weight, you have to do a combination of two things. Eat less calories and move more.
If you want to get all scientific buy a heart rate monitor, wear it all day, see how many calories you burn. Eat less than that by at least 500 calories per day.
Example, you probably move around enough without dedicating some time to working out to require 3,000 calories a day to maintain your weight. Add in a work out (lets say 500 calories burned) now you are up to needing 3,500 calories per day to maintain 300lbs.
You shouldn't deprive your body of more than 500-750 calories per day. So, if your BMR is 2600, and you actively burn an additional 500 calories per day through work, exercise, chasing the kids, etc. Then 2600 calories per day will let you lose about 1 lb per week (3500 calories per week is typically 1 lbs of fat loss per week).
Don't try getting all crazy and eating WAY less calories then you need, because then your body will feel starved and just store everything you eat because it is thinking you aren't going to feed it again and it will need to turn that food into fat stores in case you don't eat it. This is also why it is important to eat often. Your body will get used to this and think its getting some food again soon so it will not be afraid to start eating away at the bodyfat you already have to fuel your muscles while it waits for food again.